Source : Webster's Revised Unabridged Dictionary (1913)

Gentlemen's agreement \Gen"tle*men's a*gree"ment\
   An agreement binding only as a matter of honor; often,
   specif., such an agreement among the heads of industrial or
   merchantile enterprises, the terms of which could not be
   included and enforced in a legal contract.

Source : WordNet®

gentlemen's agreement
     n : a personal agreement based on honor and not legally binding